Such a policing method is already known from the article "The Spacer-Controller: an efficient UPC/NPC for ATM networks", by P. Boyer et al., ISS 7992, October 7992, Vol. 2, paper A9.3, pp. 376-320. This known method is described in particular with reference to FIG. 3 of the article and relates to the policing of an aggregate Asynchronous Transfer Mode or ATM stream in which separate substreams of first and second cells with the first cells having a higher priority than the second cells is contained. The high priority first cells are characterized by a Cell Loss Priority CLP bit equal to 0 whereas the low priority second cells have a CLP bit equal to 1. For such an aggregate ATM cell stream the ITU-TS recommendation I.371 prescribes that the high priority first cells have to be policed separately while on the aggregate stream of both first and second cells should also be policed as a whole.
As described in the mentioned article the above prescribed two policing steps are performed sequentially such that already policed high priority first cells are forwarded to the second policing step together with the unpoliced low priority second cells.
A serious drawback of this known method is that a high priority first cell may still be discarded in the second policing step regardless of its priority or, in other words, that a high priority first cell satisfactory (or conforming) in the first step may thereafter still be discarded in the second step because of the presence of low priority second cells. Such a result is clearly contrary to the establishment of a separate high priority substream.